The present invention disclosed herein relates to an optical transmission apparatus and, more particularly, to a transistor outline (TO)-CAN type optical module and an optical transmission apparatus including the same.
Typically, an optical module has had a structure of a butterfly or TO-CAN based package having a dual-in-line (DIL) pin layout. Nowadays, an optical module has been fabricated by using a legacy butterfly of a mini-DIL type or TO-CAN based miniature package that are recommended by the 10 Gbps miniature device-multiple source agreement (XMD-MSA), which is an industry standard. The legacy butterfly package not only structurally employs copper-tungsten (CuW), which has excellent thermal conductivity, as a bottom surface of the package, but also is very advantageous to impedance matching for a radio frequency (RF) operation. Thus, it has been mainly applied to an optical module having 10 Gbps or higher operation speed and allowing a cooled operation to be enabled. However, this legacy butterfly package is not proper to a low cost optical module, because packaging subsidiary materials used for the legacy butterfly type optical module are expensive and it is not easy to fabricate the legacy butterfly package. On the contrary, a TO-CAN based optical module can be fabricated in a miniature size, and a fabricating cost thereof can also be greatly lowered compared to the legacy butterfly scheme.
However, when a directly modulated laser (DML) of an edge-emitting scheme in which a light is emitted not from a top surface but from a side surface of a chip is required to use in a TO-CAN based optical transmission module, the DML chip is attached to a top surface of an ‘L’ shape block connected to a stem of the TO-CAN package, and then a light is output in a longitudinal direction of the TO-CAN package and is coupled to an optical fiber. In the above-described TO-CAN package structure, a radio frequency electrical signal penetrates through the stem of the TO-CAN package and is transferred to a DML through a protruding lead and bonding wire. A TO-CAN stem portion may be designed and fabricated in a structure where impedance matching is easily performed, but the lead and bonding wire protruding outside the TO-CAN stem have high inductances due to limited diameters and relatively long lengths. As a result, impedance values thereof are easily increased to several hundred ohms. Since this may be a main cause for seriously degrading radio frequency characteristics of the optical module having the TO-CAN package structure, the methods of obtaining excellent radio frequency characteristics are to lower inductance values of the lead and the boding wire and match the impedances in 25 ohm (for a single-ended configured DML driver) or 50 ohm (for a differential configured DML driver).
Furthermore, since the TO-stem used in the TO-CAN based optical module is required to allow laser-welding with a cover cap included in the TO-CAN to be enabled, Kovar material is used which has greatly lower thermal conductivity than CuW. In order to apply the DML chip to TO-CAN package as described above, an “L” block is additionally connected to the TO-stem and the DML chip is attached to a top surface of the “L” block. However, in this structure, a thermo-electric cooler (TEC) for cooling the DML chip is difficult to be employed and heat transfer efficiency for transferring heat generated in the DML chip gets lowered. Accordingly, the TO-CAN based optical module has been mainly applied to an uncooled operation which does not need cooling. Recently, a research on an optical module having a cooled operation scheme using a low-cost TO-CAN package is actively under development.
For the TO-CAN based optical module that a cooled operation is possible, a thermoelectric cooler (TEC) is disposed on the bottom of the TO-stem to increase heat dissipation efficiency and a reflective mirror is disposed to change a direction of a light output from the DML to a longitudinal direction of the TO-CAN package after the DML chip is attached to the top surface of the TEC. In this case, there may be two limitations from a viewpoint of high frequency operation. First, a length of the lead protruding from the TO-stem, which is as long as a height of the TEC, is further increased. Second, since a distance from an end of the lead to the DML chip is greatly increased compared to the typical TO-CAN based package structure that an uncooled operation is possible, a length of a boding wire from the lead to the DML chip becomes increased. In this case, the inductance values of the lead and the boding wire may become excessively increased, thus increasing impedance mismatch and seriously deteriorating a high frequency operation of the optical module. Thus, excellent high frequency characteristics may not be expected.